In a selctive call communication system a particular receiver is rendered operative when the carrier wave signal applied thereto contains a certain tone or set of tones to which the decoder in such receiver is designed to respond. These tones are generated by an encoder, which tones are then modulated onto the carrier wave generated by the transmitter with which that encoder is associated. In systems involving voice communication, the tone or set of tones is transmitted to unsquelch the receiver, whereupon the operator at the transmitter can speak into his microphone and the possessor of the associated receiver will hear his words. Alternatively, the system may involve nonvoice communication, wherein the receiver emits an alerting signal such as a tone when the proper tone or set of tones is applied thereto.
Usually in such systems, there is a single base station which has a transmitter, an associated encoder, and other interface equipment to enable persons to gain access to the transmitter. Such equipment may take the form of a telephone system in which a person dials a certain number to connect the telephone to the transmitter, whereupon a selected code may be transmitted. Usually a multiplicity of receivers will be associated with such transmitter. For example, a system used in a hospital would entail each doctor being furnished with a receiver designed to emit an alerting signal in response to a unique code. In this type of system, the transmitter is likely to be very expensive compared to the cost of the individual receivers. Since there is only one transmitter, the cost thereof does not render the cost of the entire system prohibitive.
It has been proposed to use selective call capability in an automatic identification system. In such system, each user, such as a vehicle, is furnished with a transmitter and an associated encoder to enable the user, not only to receive a voice message, but also to communicate with the base station and also to identify himself without so stating. When the vehicle driver wishes to communicate with the base station, he operates his push-to-talk switch and speaks into his microphone. With automatic identification capabilities, the encoder generates a signal representing that encoder, which signal is modulated onto the carrier wave. The identification signal, by way of a suitable display or otherwise, apprises the base station operator of the identity of the encoder transmitting. The value of such automatic identification is recognized and need not be delved into here. In prior systems, the information automatically sent to the base station has been limited to the identity of vehicle or driver. Such systems have not supplied additional information, such as where the vehicle is located or what is its status, etc.
Furthermore, the encoders previously available have not been sufficiently inexpensive to enable widespread use such as is necessary when many vehicles are provided with an encoder.
It is important in selective call communication systems to maximize the number of channels used in a given frequency spectrum. In other words, if the frequency spectrum for tones is, for example 500 Hz. to 3,500 Hz., it is desirable to maximize the number of channels within that range that can be utilized. Of course, the limiting factor is that the channels cannot be so close that operation of a decoder responsive to one channel will also operate a decoder responsive to an adjacent channel. Furthermore, with limiting, the encoders generate not only the specific tone, but also its harmonics, particularly its third harmonic. It is therefore important that the third harmonic of one tone in the frequency spectrum not coincide with other channels in the spectrum and, in fact, be as far removed as possible therefrom.
Since each vehicle in such a system is also provided with a receiver and an associated decoder to respond to a particular sequence of tones, it is equally desirable that the cost of the decoder be minimized.